1. Field of the Invention
The present invention relates generally to thermal imaging systems and, more particularly, to systems and methods for calibrating thermal imaging devices, such as focal plane arrays.
2. Related Art
A focal plane array (FPA), which detects infrared radiation, is well known in the art. An FPA, for example, may be formed from an array of microbolometers, with each microbolometer functioning as a pixel to produce a two-dimensional image. The change in resistance of each microbolometer due to incident infrared radiation is translated into a time-multiplexed electrical signal by circuitry known as the read out integrated circuit (ROIC). The combination of the ROIC and the microbolometer array is commonly known as a microbolometer FPA or microbolometer infrared FPA. Microbolometers and FPAs are described in further detail in U.S. Pat. Nos. 5,756,999 and 6,028,309, which are herein incorporated by reference in their entirety.
Thermal imaging device performance, such as with a FPA for example, is typically degraded due to non-uniform responses among the individual microbolometer detectors to uniform incident infrared radiation. Factors contributing to the performance degradation include variations in the infrared radiation absorption coefficient, resistance, temperature coefficient of resistance (TCR), heat capacity, and thermal conductivity of the individual detectors. Because the magnitude of the non-uniformity can be substantially larger than the magnitude of the actual response due to the incident infrared radiation, various techniques are typically required to compensate for the non-uniformity and acquire the portion of the signal representing the incident infrared radiation.
FIG. 1 is a graph of pixel output (e.g., output voltage) for two microbolometers within an FPA as a function of photon flux (i.e., received incident infrared radiation). As lines 102 and 104 illustrate for the two corresponding microbolometers, the pixel outputs for a certain initial level of photon flux (e.g., at point 106) differs by a certain amount of pixel output offset. Furthermore, the amount of gain exhibited by the two microbolometers varies over a range of photon flux (e.g., between points 106 and 108), as indicated by the difference in the slope of lines 102 and 104.
Typically, the offset and gain of each infrared detector in the FPA is calibrated so that a more uniform response is obtained from the microbolometer FPA over the desired range of photon flux. For example, as shown in FIG. 2, the initial offsets for the two microbolometers are calibrated at point 106 to remove the non-uniform offset. The gain is then normalized, as shown in FIG. 3, for the two microbolometers over the photon flux range defined by points 106 and 108 to produce a more uniform response. Further details of calibration procedures may be found, for example, in U.S. Pat. Nos. 5,756,999 and 6,028,309.
Thermal imaging devices are typically periodically calibrated, such as upon power-up or at certain intervals during use, to minimize the non-uniform response from the FPA. For example, the FPA of the thermal imaging device may be calibrated over two or more levels of photon flux by inserting into the optical path a calibration flag (i.e., an optical obscuration). The temperature of the calibration flag is raised or lowered to provide the desired level of photon flux for the FPA. When the calibration flag reaches the required temperature and is inserted into the optical path of the thermal imaging device, the FPA takes one or more data frames or snapshots of the calibration flag to calibrate its response at that temperature. The temperature of the calibration flag can then be changed and the calibration process repeated at the new temperature. The data collected at the calibration points can then be used to calibrate the FPA to provide a more uniform response, as discussed above.
The calibration flag is typically coupled to a thermoelectric cooler (TEC), which is a small heat pump that heats or cools the calibration flag to the desired temperature (i.e., desired level of photon flux). The TEC is coupled to a heat sink, which is used to help maintain the desired temperature and prevent temperature elevation drift or overheating of the TEC or calibration flag. In some implementations, a fan may be used to further aid in maintaining the desired temperature range. A small motor or servo is typically used to translate or rotate the calibration flag into the optical path when calibration is desired.
One drawback of this technique is the significant time delay between calibration points due to the time required by the TEC to heat or cool the calibration flag from one temperature calibration point to the next. For example, one or more minutes may be required by the TEC to transition or slew the calibration flag between calibration temperature points. Another drawback is that the fan, servo, and heat sink detrimentally add to the size, weight, cost, and complexity of the thermal imaging device. Furthermore, the fan and servo are an additional power draw and may contribute undesired electromagnetic interference. As a result, there is a need for improved techniques for providing calibration for thermal imaging devices.